This invention relates, according to an aspect thereof, to a method for diagnosing a selective catalytic reduction (SCR) catalyst of an exhaust aftertreatment system of an internal combustion engine, wherein said exhaust aftertreatment system comprises a SCR catalyst, an reductant injector arranged upstream of said SCR catalyst for injecting a reductant into an exhaust gas, and an exhaust gas sensor arranged downstream of said SCR catalyst, which sensor being cross-sensitive to nitrogen oxide (NOx) and ammonia (NH3) present within the exhaust gas leaving said SCR catalyst.
The invention is applicable for example on vehicles provided with SCR catalyst, such as working machines within the field of industrial construction machines, in particular wheel loaders and articulated haulers, but also other vehicles such as a truck or bus comprising a compression ignition engine configured to be powered with diesel or similar fuel types.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has a high efficiency of and is one of the best converters of fossil energy. NOx emission concentration is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable. Moreover, a suppression of NOx formation by internal means (air/fuel ratio) has the tendency to cause an increase in particulates, known as the NOx-particulates trade off, Furthermore, an excess of oxygen in the exhaust gas from a diesel engine prevents the use of stoichiometric 3-way-catalyst technology for reduction of NOx as is used in gasoline engine cars from the late 80-ties.
Reducing the oxides of nitrogen (NO and NO2, referred to as NOx) and particulate matter (PM) in exhaust gases from a diesel engine has become a very important problem in view of the protection of environment and the saving of finite fossil energy supply.
Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas. In this regard Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency. Urea based SCR catalysts use gaseous ammonia as the active NOx reducing reductant. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into hydro cyanic acid (NHCO) and gaseous ammonia (NH3), which is then used to convert NOx.
However, in such systems, urea injection levels have to be very precisely controlled. Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip, In a typical urea-based SCR catalyst system, the amount of urea injected is in proportion to the exhaust gas NOx concentration that represents a trade-off between maximum NOX conversion and minimum ammonia slip.
NOx conversion efficiency of an SCR catalyst is improved in the presence of adsorbed ammonia within the SCR catalyst, wherein the level of adsorbed ammonia is denoted SCR catalyst ammonia storage level. NOx conversion efficiency is improved in a steady state condition by the ammonia buffer within the SCR catalyst. Moreover, NOx conversion efficiency is particularly improved during transient conditions by the ammonia buffer, when reductant injection may not follow the transient NOx emission level correctly. In this situation, adsorbed ammonia, within the SCR catalyst assures sufficient continuous NOx reduction to respect regulatory emission levels. However, if too much ammonia is stored in the catalyst under certain operating conditions, such as high temperatures, some of the stored ammonia in the catalyst may desorb and slip from the catalyst or to be oxidized to NOx and thereby reducing the overall NOx conversion efficiency.
For the purpose of efficient and regulatory fulfilling NOx emission reduction, it is desirable to determine the SCR catalyst performance and condition, and in particular the NH3 storage capacity of the SCR catalyst.
Document U.S. 2010/0326051 shows an operating and diagnostic method, wherein the total converted ammonia amount which is stored in the SCR catalytic convener may be calculated. This diagnostic method however suffers from relatively low accuracy with respect to the true storage capacity of the SCR convener.
There is thus a need for an improved method for diagnosing a SCR catalyst removing the above mentioned disadvantages.
It is desirable to provide an inventive method for diagnosing a SCR catalyst where the previously mentioned problem is at least partly avoided.
The invention concerns, according to an aspect thereof, a method for diagnosing a selective catalytic reduction (SCR) catalyst of an exhaust aftertreatment system of an internal combustion engine, wherein said exhaust aftertreatment system comprises a SCR catalyst, an reductant injector arranged upstream of said SCR catalyst for injecting a reductant into an exhaust gas, and an exhaust gas sensor arranged downstream of said SCR catalyst, which sensor being cross-sensitive to nitrogen oxide (NOx) and ammonia (NH3) present within the exhaust gas leaving said SCR catalyst.
The inventive method comprises the steps of ensuring that a SCR catalyst NH3 storage level is substantially below the current maximal NH3 storage capacity of said SCR catalyst; initiating over-injection of reductant by said injector; stopping reductant injection upon registering, by said exhaust gas sensor, an increasing NOx+NH3 emission level and an NOx+NH3 emission level exceeding a predefined threshold value, and when the reduction injection is stopped recording an output signal of said exhaust gas sensor until an indication of a minimal or negligible NH3 storage level is determined; and calculating, a SCR catalyst NH3 storage capacity on the basis of said recorded output signal.
The inventive method provides improved accuracy of the calculated NH3 storage capacity of the SCR catalyst because the level of ammonia slip at begin of the recording of the output signal of said exhaust gas sensor is very small, and will therefore only distort the calculated NH3 storage capacity value to a minimal extent. This very small level of ammonia slip can be detected by the inventive method due to the careful preparation of the recording, namely ensuring that the level of stored ammonia is relatively low and subsequently initiating over-injection if reductant. An important aspect of the inventive method is early detection of ammonia slip, which occurs upon reaching a high level of ammonia storage in the SCR catalyst. It is known to install an additional ammonia sensor downstream of the SCR catalyst for detecting ammonia slip, but an additional ammonia sensor results in higher cost, more complex system with higher risk of malfunction. The early detection of ammonia slip is according to the invention accomplished by continuously monitoring the output signal from the exhaust gas sensor. No additional ammonia sensor is consequently required. The over-injection of reductant will lead to ammonia slip when the ammonia storage level is sufficiently high, but due to the initial low level of stored ammonia in the SCR catalyst, it can be established that the initial output signal of the exhaust gas sensor corresponds to the NOx emission level only, and that no ammonia slip distorts said NOx emission level. Knowing that the output signal of the exhaust gas sensor corresponds only to NOx emission, it can be concluded, that any increase in the output signal of the exhaust gas sensor must result from ammonia slip, based on constant NOx emission level entering the SCR catalyst, and constant exhaust gas temperature. Consequently, a very early detection of ammonia slip can be established, without the use of a dedicated ammonia sensor, thereby facilitating calculation of the SCR catalyst NH3 storage capacity with high level of accuracy.
Also, using an indication of a minimal or negligible NH3 storage level as end criteria of the output signal recording of the exhaust gas sensor further improves accuracy of the NH3 storage capacity calculation. The ammonia storage capacity calculation is based on the assumption that all ammonia in SCR catalyst have been converted, i.e. zero NH3 storage level. Evidently, if some amount of adsorbed ammonia still remains within the SCR catalyst, the calculated NH3 storage capacity will be distorted to a certain extent. Depending on SCR catalyst type, operating temperature, etc., the time to completely empty the SCR catalyst of ammonia may take relatively long time. A compromise between the accuracy of the calculated NH3 storage capacity and time for performing the diagnose is thus established. From above, it is clear that an indication of a minimal or negligible NH3 storage level will deliver a highly accurate calculation of the NH3 storage capacity of the SCR catalyst.
Knowing accurately the NH3 storage capacity of the SCR catalyst, not only can the reductant dosage level be controlled more optimally, but an indication of the status of the SCR catalyst is derived. As previously mentioned, high NH3 storage capacity of the SCR catalyst can to a greater extent convert transient NOx emission levels. Upon detecting a SCR catalyst with a relatively small NH3 capacity, this may indicate need of replacement or repair of the SCR catalyst.
The method may further comprise the step of providing the exhaust aftertreatment system with an additional exhaust gas sensor arranged upstream of said reductant injector, which additional exhaust gas sensor being configured to measure the level of NOx present within the exhaust gas entering said SCR catalyst. By providing a NOx-sensible sensor upstream and downstream of the SCR catalyst, the conversion efficiency can be directly determined, thereby facilitating improved and simplified control of reductant injection level, as well as continuous monitoring of the NOx emission conversion efficiency. Without the additional exhaust gas sensor the NOx emission level of the exhaust gas entering the SCR catalyst can be estimated based on for example a stabilised NOx emission level of the exhaust gas leaving the SCR catalyst upon stopped reductant injection, in which condition is can be assumed that NOx emission level of the exhaust gas entering the SCR catalyst equals NOx emission level of the exhaust as leaving the SCR catalyst.
The step of ensuring, that said SCR catalyst NH3 storage level is substantially below the current maximal NH3 storage capacity of said SCR catalyst may be realised by reducing injection of reductant until a NOx level downstream of said SCR catalyst as provided by said exhaust gas sensor is above 50%, specifically above 75%, more specifically above 90% of a NOx level upstream of said SCR catalyst as provided by said additional exhaust gas sensor. During normal operation of a modern SCR catalyst based exhaust aftertreatment system, NOx conversion efficiency is substantially above 70%. Accordingly, the NOx level downstream of said SCR catalyst is normally substantially below 30% of a NOx level upstream of said SCR catalyst.
Consequently. when a NOx level downstream of said SCR catalyst is above 50% of a NOx level upstream of said SCR catalyst, this is a strong indication that the SCR catalyst NH3 storage level is depleted, because otherwise the remaining NH3 in the SCR catalyst would assure a significantly reduced NOx level downstream.
The step of ensuring that said SCR catalyst NH3 storage level is substantially below the current maximal NH3 storage capacity of said SCR catalyst may alternatively be realised by reducing injection of reductant until a rate of change of the output signal of said exhaust gas sensor decreases down below a predefined second threshold value. Upon reducing reductant injection to a sufficient extent, stored ammonia within the SCR catalyst will eventually be converted, and the NOx level downstream of the SCR catalyst will start to increase because insufficient ammonia is provided, for efficient NOx conversion. The rate of change of the output signal of said exhaust gas sensor will thus initially increase along with increased NOx level. The NOx level downstream of the SCR catalyst will eventually stabilise, and the rate of change of the output signal of said exhaust gas sensor will decrease again, eventually down below said predefined second threshold value.
The method may further comprise the step of starting the sequence of method steps by initiating engine operation at a steady state point with respect to NOx emissions reaching said SCR catalyst, and keeping said engine steady state point operation until the end of said recording of said output signal of said exhaust gas sensor. Constant NOx level upstream of SCR catalyst allows the use of a single downstream exhaust gas sensor to be used, and supports the maintenance of a stable exhaust gas temperature.
The method may also comprise the step of further waiting for a predefined time period after initiating engine operation at a steady state point, such that the exhaust gas properties and exhaust aftertreatment system can stabilise.
The method may further comprise the step of providing isothermal exhaust gas conditions at the inlet of said SCR catalyst before initiating said over-injection of reductant. or at least before stopping reductant injection upon registering said increasing NOx+NH3 emission level and an NOx+NH3 emission level exceeding the predefined threshold value. SCR catalyst ammonia storage capacity is normally to a large degree dependent on the temperature of the SCR catalyst, and isothermal exhaust gas conditions at the inlet of said SCR catalyst thus supports a stable SCR catalyst temperature, such that an accurate SCR catalyst diagnose can be determined for a particular temperature level.
A control unit associated with the exhaust aftertreatment system may be configured to control reductant injection by means of a reductant dosing model, and a reductant dosage rate corresponding to said over-injection may be determined by multiplying a dosage rate as provided by said reductant dosing model with an overflow coefficient. The reductant dosage model normally provides the dosage rate that is considered to result in most efficient NOx conversion of the SCR catalyst, Upon multiplying said dosage rate with an overflow coefficient, such as for example 2.0, an increased level of reductant dosage is accomplished. If the overflow coefficient is too high, unreacted liquid reductant may accumulate in the exhaust gas pipe, and if the overflow coefficient is too low, an ammonia-slip catalyst may have sufficient capacity to convert the entire ammonia-slip leaving the SCR catalyst, such that no ammonia-slip can be detected by the exhaust gas sensor installed downstream the combined SCR catalyst and ammonia-slip catalyst.
The increasing NOx+NH3 emission level and an NOx+NH3 emission level exceeding the predefined threshold value may be registered when SNOX_OUT>SNOX_MIN, where SNOX_OUT denotes the current output signal of said exhaust gas sensor, SNOX_MIN denotes an updated minimum exhaust gas sensor output signal recorded during said recording, and K denotes a predefined multiplication factor. This method of determining begin of ammonia-slip provides a fast and reliable identification of start of ammonia-slip, By constantly updating the minimum exhaust gas sensor output signal (SNOX_MIN), the downstream NOx value is allowed to initially sink upon initiation of reductant over-injection, and no predetermined quantitative threshold value is required.
An indication of a minimal or negligible NH3 storage level may be determined when the NOx level as measured by said exhaust gas sensor has reached a predefined percentage level, such as 80%, of the NOx level as measured by said additional exhaust gas sensor, or alternatively, when a predetermined time period has passed after the NOx level as measured by said exhaust gas sensor has reached a specific percentage level, such as 80%, of the NOx level as measured by said additional exhaust gas sensor. The accuracy of the calculated SCR catalyst ammonia storage capacity increases with recording time, but since downstream NOx level somewhat asymptotically approached upstream NOx level, the difference in downstream and upstream NOx levels become more and more insignificant to the final calculated storage capacity. A criteria for ending registering of NOx levels is thus required, and is preferably set to deliver an accurate result without making the diagnose method too time consuming. A single sequence of the method steps may typically, large depending on SCR catalyst temperature and type, take around between 1-10 minutes, and a complete diagnose including several NH3 storage capacity calculations conducted at different SCR catalyst temperatures may typically take around between 10-30 minutes.
An indication of a minimal or negligible NH3 storage level may be determined when a rate of change of the output signal of said exhaust gas sensor has decreased below a predefined third threshold value. Upon stopping or reducing reductant injection, any stored ammonia within the SCR catalyst will eventually be converted, and the NOx level downstream of the SCR catalyst will start to increase because insufficient ammonia is provided for efficient NOx conversion. The rate of change of the output signal of said exhaust gas sensor will thus initially increase. The NOx level downstream of the SCR catalyst will eventually stabilise, and the rate of change of the output signal of said exhaust gas sensor will decrease again. When the reductant injection is completely stopped, the NOx level downstream of the SCR catalyst will stabilise at a level corresponding to the NOx level upstream of the SCR catalyst.
The calculation of said SCR catalyst NH3 storage capacity is based on integrating, preferably from the start to the end of said recording, a difference between the recorded NOx level as measured by said exhaust gas sensor and a constant value corresponding to the recorded. NOx level as measured by said exhaust gas sensor at the end of said recording. During said recording, reductant injection is stopped, and since the ammonia slip due to said over-injection was detected very early, as mention above, substantially zero ammonia slip will be included in said recording. From this, it is clear that the output signal of the exhaust gas sensor completely corresponds to the NOx emission level only, and not due to any ammonia slip. Furthermore, by estimating with good accuracy the amount of converted NOx during said recording, the level of stored ammonia within the SCR catalyst can be derived. For best accuracy, the complete recording, i.e. from the start to the end, should be used. The amount of converted NOx during said recording could be derived by taking the difference between NOx level upstream and downstream of the SCR catalyst. However, when no upstream NOx level information is available, the more or less stabilised downstream NOx level may represent the upstream NOx level.
The method may further comprise the step of recording also an output signal of said additional exhaust gas sensor simultaneously with said recording of said output signal of said exhaust gas sensor. As a consequence, the calculation of said SCR catalyst NH3 storage capacity may be based on integrating the difference between the recorded NOx level as measured by said exhaust as sensor and the recorded NOx level as measured by said additional exhaust gas sensor. According to this configuration, there is thus no need to make any estimate of the upstream NOx level, as previously discussed.
The calculation of said SCR catalyst NH3 storage capacity is preferably further based on a mass or volumetric flow rate of the exhaust gas during; said recording, a NO/NO2 ratio of the exhaust gas entering said SCR catalyst, and a predictive NH3-NO/NO2 reaction model for said SCR catalyst, and a SCR catalyst exhaust gas temperature.
The method may further comprise the step of repeating the sequence of method steps for diagnosing said SCR catalyst at a different isothermal condition for acquiring an improved diagnose of the SCR catalyst.
The SCR catalyst may jointly with a NH3 slip catalyst form a single unit. The NH3 slip catalyst would then be configured to remove unreacted ammonia leaving the SCR catalyst up to a certain level. The exhaust gas sensor is then arranged downstream of said unit, i.e., downstream of said ammonia slip catalyst.
The method may further comprise the step of comparing said calculated SCR catalyst NH3 storage capacity with earlier calculated SCR catalyst NH3 storage capacity, or other type of reference data relating to SCR catalyst NH3 storage capacity.